My friend has a 2004 X3 3.0i and has a problem with a high pitched whistling noise coming from somewhere on the top part of the windscreen near the rear view mirror at speeds between 80 - 95mph. He says it is more apparent when the weather is hot (20 degrees plus) and that washing the windscreen stops it temporarily. Has anyone else experienced this problem and knows what the cause is?

Also the screen appears to be the original.

Thanks